Quote Originally Posted by Farecry View Post
Why is this spell considered so good when you lose a turn for it?
Well, first off, it's an invocation and not a spell. So there is no spell slot cost, it's just something you get to do once per short rest.

Secondly, I don't know who considers it "so good" but it's not useless.

Third, because it's a reaction that happens when you take damage, it lets you play closer to the edge and actually SAVE turns that might otherwise be spent on healing or defensive maneuvers. It is basically protection against novas.

Imagine how frustrating it would be to be the enemy (I almost said "DM" but the DM isn't the enemy) of a party of four warlocks, all with the Tomb of Levistus. Even if you manage to get one of them down to 11 HP and are threatening to insta-kill him with a critical hit or breath weapon, he doesn't retreat or fall back or anything. He may not even bother to pause and re-cast Armor of Agathys. Instead he just keeps on attacking you, figuring that when you try to strike a killing blow he will just turn to ice and shrug it off. Novas don't work against these guys. Even if you outright Meteor Swarm the whole party, they will probably just negate it, and now you've wasted your action AND a 9th level spell slot in exchange for... forcing them to waste their action. Net win for the warlock side.

Combine Tomb of Levistus with a decent source of healing (like Healing Spirit or Aura of Vitality) and the warlock becomes (somewhat) immune to BOTH novas and attrition.